One method of dealing with excess noise generated by a specific source is to treat the noise at the source. This is typically accomplished by adding acoustic damping structures (acoustic treatments) to the structure surrounding noise source. One particularly problematic noise source is the jet engines used on most passenger aircraft.
Open cell honeycomb core is a popular material for use in aircraft and aerospace vehicles because it is relatively strong and lightweight. For acoustic applications, a goal has been to incorporate acoustic material into the honeycomb structure. For instance, historically, acoustic material has been inserted into the honeycomb cells. In historical cases where expandable honeycomb core has been used, the acoustic material has been inserted into the honeycomb cells post-expansion. In historical cases where corrugated honeycomb core has been used, the acoustic material has been inserted into the honeycomb cells post bonding or welding of the stack. In both aforementioned cases, the acoustic material may be individually inserted into each honeycomb cell and may remain in place by friction locking or by applying an adhesive to the acoustic material.